In the commercial fields of reprography, an electrostatic image has been widely developed by a magnetic brush development using two-component magnetic developers. The two-component magnetic developers widely used are those which are made of mixtures of magnetic carriers consisting of iron powder or sintered ferrite particles and toner particles made of dispersions of ingredients, such as colorants, charge controlling agents and the like, in fixing resins.
When the two-component magnetic developer has been employed over a long term, the magnetic carrier is gradually covered with the resin of the toner particles on the surfaces thereof, presenting the so-called "spent" problem. In the developer, the magnetic carrier is generally low in electric resistance. In order to obtain an image of high quality, there is a demand for a combination of a high resistance carrier and a low resistance toner.
To solve these problems, magnetic carriers coated with a resin on the surface thereof have been proposed and have now been in use. A number of proposals have been made on the type of resin coating. For instance, Japanese Patent Publication No. 58-9946 proposes the resin coating for carrier which is made of a combination of from 5 to 30 wt % of a melamine resin and the balance of an epoxy resin, an acrylic resin or an alkyd resin. Moreover, in Japanese Laid-open Patent Application No. 62-262057, there is described the coating of the carrier surface with a resin obtained by curing reaction between a thermoplastic resin having unreacted hydroxyl groups and alkoxylated melamine resin.
The resin coating of carrier particles should meet two requirements which stand opposite each other, i.e. strong adhesion of the resin to the surface of the carrier particles, and no mutual adhesion of resin-coated carrier particles.
The resin coating containing a melamine resin or an alkylated melamine resin is advantageous in that when a carrier and a toner are mixed, the coating causes the carrier to be charged positively and, correspondingly, the toner to be negatively charged, coupled with another advantage that the resin may act as a curing agent for other reactive resins.
However, where the conventional alkylated melamine resin-containing coating is applied onto the carrier surface, the resin coating layers are fusedly combined together thereby causing the carrier particles to be bonded. If the bonded particles are broken into pieces, the carrier surface inevitably becomes irregular owing to the breakage of the resin coating layer. This makes sit difficult to form a uniform resin coating layer on the carrier surface. In addition, deposition of the toner on the irregular portions will produce a spent phenomenon, causing the life of the carrier to be shortened.